304 



SYSTEMATIC ZOOLOGY. 



sclerotic coat, which is usually white (the white of the eye 

 is part of it), and which may be cartilaginous or may 

 even have bone deposited in it, as in many reptiles and 

 birds. In front this layer becomes perfectly transparent, 

 and is there known as the cornea. Inside of the sclerotic 

 is found a densely black layer (choroid), and still within 

 this the transparent retina, the outer portion of which is 



FIG. 1 13. Diagram of vertebrate eye. c, choroid i| -^iris; I, lens ; n, optic nerve ; 



r, retina; s, sclerotic. 



imbedded in the choroid. In front the choroid is con- 

 tinued into the iris, a circular muscle with an aperture, 

 the pupil, in its centre. This iris, which is colored, regu- 

 lates by its enlargement and contraction the amount of 

 light which is admitted to the visual parts of the eye. 

 Back of the iris, and held in position by a circular muscle 

 and ligament, is the transparent lens. In front of this 

 lens is a watery fluid (aqueous humor) , while behind it and 

 between it and the retina is the somewhat denser vitreous 

 humor. 



The optic nerve enters the eye from behind, passing 



